excavation safety manual

E X C A V A TI O N S A F E T Y

The University of Texas at Arlington - Susan Harwood Program Grant



Excavating is one of the most hazardous forms of construction activitydue to the possibility of cave-ins. While the number of excavationaccidents is small when compared to the total number of accidents

occurring in all types of construction activity, they occur more frequentlythan in other types of construction, and are far more likely to result in aserious injury or fatality. For these reasons OSHA has, historically, paidclose attention to excavation work, elevating it to a “special emphasis”program in 1985, and making it the first construction standard they totallyrevised (in 1989).

Research done by OSHA during the course of the revision of the Standard inthe 1980’s showed that the fatality rate in excavation work was as much as112% greater than the rate for construction in general (.508 per 1000employees in excavation work as opposed to .248 per 1000 for constructionin general). In other words, an employee engaged in excavation activity wastwice as likely to be killed as those engaged in other forms of constructionwork. Of special interest was a California OSHA report specifically relatedto cave-ins that revealed that while the ratio of lost time accidents tofatalities for all California industries was 250 to 1, the ratio for excavationwork was 17 to 1. In other words, a person engaged in excavation work wasnearly 15 times more likely to die from an accident (typically a cave-in)than in any other form of industrial activity.

Given the contributing factors involved in a cave-in, the numbers are notsurprising. A cubic foot of soil weighs one hundred pounds on average.That would make a cubic yard, the capacity of most backhoe loader frontbuckets, twenty seven hundred pounds; the weight of a mid-sizedautomobile. A person working in an unprotected excavation eight feet deepon a sewer main could easily have 3-6 cubic yards (8,100-16,200 lbs.) buryhim in a cave-in. The force of that impact could snap bones like matchsticksand crush soft tissue like gelatin. Even if the employee were to survive theinitial impact, the weight of the soil on the chest and abdomen wouldtypically prevent them from breathing, resulting in what is known as“compression asphyxia”; the leading cause of death in cave-ins.Additionally, there are those who survive the initial impact and are caught ina position where they can breathe long enough to be rescued, but sometimesdie after extrication from complications resulting from trauma, or from thesudden restoration of blood circulation in what is known as “crushsyndrome”.

OSHA Research

INTRODUCTION

Introduction toExcavation Safety

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As troubling as the numbers are, what is even more troubling is that, insome cases, they report totals that may be less than the true numbers. Thedatabases are all insufficient to some degree. Small employers engaged inexcavation activity, such as plumbing and light utility constructioncompanies, may be exempt from the Bureau of Labor Statistic reportingrequirements. Employees of political subdivisions of the states, who areoutside federal OSHA jurisdiction, might not be counted. Employers whoare not carrying Workers Compensation Insurance may not show up in theWorkers Comp databases. Finally, even death certificates may fall short ofproviding the complete number of employees killed by cave-ins. Fatalitiesoccurring as a consequence of cave-ins are often attributed to trauma,compression asphyxia, or drowning (if the event occurs in a submergedexcavation) on the death certificate. As a consequence, the number of deathsreported by the standard databases may be less than the actual numbers. Thedatabases have improved over the years, however. The numbers, whileapproximate, provide a reasonably reliable picture because the degree ofaccuracy or error tends to remain constant over the long term.

The data are certainly reliable enough to demonstrate trends and the impactof such events as the passage of affecting legislation. The studies conductedby OSHA during the revision of the Excavation Standard and during aspecial investigation in 2004 by the Office of the Directorate ofConstruction, as well as the numbers generated by other agencies alsotracking construction accidents have demonstrated that the number of cave-in fatalities have dramatically decreased since the passage of the revisedexcavation standard (Figure 1).

OSHA Investigated Trench Related Fatalities

Figure 1

What are the truenumbers?

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Further, the research has shown that now, as was the case thirty years ago,the accidents occur to employees who are either working in excavationswithout any protective system, or one where the protective system is clearlyinadequate (Figure 2).

The IncidentThe protective measure in this fatality was:

Figure 2

Source: OSHA Construction Directorate Office Review of 2003 Trench Fatalities

OSHA found that many, particularly the guilty, attributed the highfrequency of cave-in fatalities to unstable soils and high volumes ofconstruction activity. However, OSHA has been able to demonstrate that,even as far back as the 19th century, proven methods existed to protectworkers in the poorest of soils. Further, work by those such as Dr. DennisPerrotta of the Texas Department of Health, who undertook a study of cave-in fatalities in Texas in the 1980’s, determined that the highest rate of cave-in fatalities occurred during periods of the lowest construction activity ratherthan during construction “boom” periods (Figure 3). A review of deathcertificates in Texas for 1976-1985 identified 93 fatalities resulting from83 excavation cave-ins among male workers in that state. Thirty-fivefatalities occurred in 30 incidents during the first five years (1975-1980),and 68 fatalities occurred in 51 incidents during the last five years(1981-1986). This is a 66 percent increase in the number of such deathsduring the second five years.

MMWR Vol. 35 No. 9 Dennis Perrota, Ph.D.DirectorEpidemiology DivisionTexas Department of Health

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Trench Fatalities in Texas

“Boom” Years Recession Years Figure 3

What history has clearly shown is that the real culprit is a triumvirateconsisting of a lack of training leading to poor decisions concerningexcavation safety practices, exacerbated by pressure to meet schedule andbudgetary constraints. Simply stated, ignorance, stupidity and an unrealisticconcern for time and money lead to dangerous corner cutting in order to geta job done cheap and fast. In the worst cases, some offenders considered therisks acceptable and the expense of the fines, accidents and fatalities the costof doing business.

By promulgating a new excavation standard, OSHA took direct aim on theproblems of the past. OSHA eliminated the confusion that existed betweentrenches and other excavations by defining excavations to include trenches,thereby making all excavation requirements applicable to trenches. A singlesoil classification system was generated by the National Bureau ofStandards (known in the OSHA Standard as Appendix A) and utilized in thesloping and shoring appendices as well as adopted by manufacturers ofexcavation safety equipment and excavation design engineers. Aluminumhydraulic shoring, which had gained significant popularity since thepromulgation of the original standard, was included. Additionally, OSHApermitted the use of tabulated data, manufacturers’ tabulated data, or site-specific designs provided by registered professional engineers as alternativesto the solutions provided in the standard. This not only provided a means forutilizing systems not included in the standard, such as sheet piling and sliderails, but also allowed for the development and utilization of future systemswithout requirement for a modification of the standard.

OSHA Takes Aim

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Of critical importance was the role assigned to a special person on thejobsite; a person OSHA calls the “Competent Person”. While there are manyelements and roles involved in the creation of a safe work program, one ofthe most critical is that of the on-site supervisor. OSHA knew that if thatsupervisor cared as much about the safety of their employees as they didgetting the pipe installed, the job would be a safe one. With the objective ofputting the right person in that role, OSHA required employers to provide acompetent person, which is defined as one, “…who is capable ofrecognizing existing and predictable hazards in the surroundings, and whohas authorization to take prompt, corrective measures to eliminate them” onthe excavation site. In the preamble to the standard, they further requiredthat the competent person receive specific training in and be knowledgeableabout: soils analysis, the use of protective systems, and the requirements ofthe standard.

It is interesting to note that in the OSHA Construction Directorate study ofexcavation fatalities occurring in 2003, the competent person was not on thesite 86% of the time (figure 4). Clearly, the presence of a competent personwhen workers are at exposure is a critical element of a safe work program.

Was the competent person on the site?

Figure 4

In the optimal model, the on-site supervisor would be assigned the role ofthe competent person and would be supported by the excavator operator,who could assume that role if the competent person had to leave the site orwas preoccupied with other duties. The employees who would be atexposure would receive training in the recognition and avoidance ofhazards, as well as the specific instruction of their protective system(s) andthe safe work protocols which would allow them to work with the operatorand supervisor to ensure that the work was done in a safe, compliantmanner. For example, an employee working in an excavation who noticedthat ground water was beginning to seep in would know to get out and callthe problem to the attention of the competent person, who would

The CompetentPerson

An Optimal Model

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immediately re-inspect the excavation and take any corrective action, suchas increasing the slope of the excavation walls and pumping out the water,before anyone was allowed to re-enter.

The spectrum of variations on the model is obviously quite broad; a twoman plumbing company repairing water main breaks differs greatly from atwo hundred man highway/utility contractor for whom excavation is onlypart of the business. If multiple employers are involved on the same site, theissue is further complicated. None-the-less, OSHA expects each employer totake care of their own employees by providing the requisite training and acompetent person to ensure their safety. In the case of a multi-employerworksite, OSHA would further expect coordination and cooperationamongst the employers involved to ensure that all employees were workingsafely during their particular part of the construction process.

These dramatic revisions in the OSHAStandard clearly impacted the number ofexcavation fatalities in the nation. Those whowished to comply finally had a standard thatprovided adequate resources and flexibility toallow them to do so. Those who did not,suddenly found that the excuses for not doingso in the past had been swept away.

This, in turn, permitted the punishment fornon-compliance to be more easily and severelyapplied. OSHA’s revised penalty structure, incombination with excavations special emphasisstatus, allowed OSHA to assess fines of up to$70,000.00 per person at exposure in the most egregious cases. Finestotaling hundreds of thousands of dollars subsequently began to be assessed.While fines in that range couldeasily devastate the finances of a non-compliant employer, they often provedto be only the tip of the iceberg. Increased workers compensation insurancerates resulting from the accident could severely impact a contractors abilityto bid competitively. Further, bonding companies or safety conscious clientswho reviewed contractors safety records as part of the bid or qualificationprocess might exclude those with problematic histories. Finally, the non-compliant might find themselves squarely in the sights of two groupsfollowing closely on OSHA’s heels who carried large clubs of their own;local criminal prosecutors and plaintiff’s attorneys.

Variations in theModel are Broad

Punishment forNon-Compliance

Dramaticrevisions

to the OSHAStandardclearly

impacted thenumber ofexcavation

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The combined efforts of OSHA, state legislators, local prosecutors andplaintiffs attorneys bear a notable testament in Texas. Texas is an interestingstate, but sometimes for the wrong reasons. One of them is that Texas hasled the nation in most categories of accidental deaths at one time or another.In some years during the 1980’s, the rate of cave in fatalities in Texas wasapproximately eight times the national average. Houston typically laidclaim to first place, followed closely by the Dallas/Ft. Worth and Austin/SanAntonio metroplexes. In 1987, a Texas congresswoman named ErnestineGlossbrenner undertook correcting the problem by passing legislationaffecting the manner in which excavation work was contracted andconducted. Ms. Glossbrenner’s investigations had determined thatapproximately two thirds of the trenching fatalities had occurred on publicsector utility projects. The reason was that, in far too many cases, the mostirresponsible owners were married to the most irresponsible contractorsunder the vow of low bid. While a municipal contract might well require acontractor to abide by all federal, state and local statutes, the practical factwas that most city inspectors had little or no training in the requirements ofthe OSHA standard and were often told by city managers that safety issueswere to be left to the contractor and OSHA. Insofar as the number ofcompliance officers in any OSHA office was generally exceeded by thenumber of counties they had to cover, the chances of an offender beingcaught and fined prior to the accident by OSHA were slim, and that fact waswell known. Those who were prone to ignore safe work requirements did soand what resulted was a situation where unscrupulous contractors gained anunfair bidding advantage over their reputable counterparts by leaving thecosts of safety equipment, training and programs out of the equation.Tragically, they were often rewarded with tax dollars for doing so.

Ms. Glossbrenner sought to reverse the course by passing House Bills 662and 665 in 1987. HB 662 required that, on construction projects where thedepth of the excavation would exceed 5 feet, both the bid documents andcontract must contain detailed plans and specifications which met the OSHAStandard, and that those plans must include a pay item for trench safety. Itwas believed that the use of a plan would ensure worker safety, as well asprovide inspectors with a guidance tool for enforcement, and that the payitem would eliminate any contractor objections to the use the plan.Contractors were, after all, in the business of making money. Identifyingtrench safety as a pay item in the contractboth declared its necessity and rewarded the contractor for taking measuresto ensure it. HB 665 allowed for the inclusion of the bidrequirement of HB 662 in building codes. The bills were enacted inSeptember of 1987, a year in which fifteen people died in cave-ins. In 1988,the number dropped to two. In 1989, the number was zero.

Combined Efforts inTexas

Glossbrenner Seeksto Reverse UnfairBidding Advantages

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Despite their impressive record, the bills were not without problems.Questions such as who was going to design the plan and whether the onlyperson submitting it should be the successful bidder quickly arose. Liability,a matter everyone wished to attempt to foist off on someone else, became asubject of great concern. In an attempt to resolve these and other issues, Ms.Glossbrenner enacted HB 1569 in the fall of 1989. That bill eliminated therequirement for trench safety plans, making them a matter of option. Thebill did retain the requirement for a pay item for trench safety, a reference tothe OSHA Standard, a copy of any available geotechnical information andany specific shoring requirements of the political subdivision, but therequirement for plans was eliminated. The created much cause for concernamong those who supported the use of trench plans. They were fearful thatthe State was poised to return to the dark days and fatality rates prior to1988. Fortunately, that did not prove to be the case, for several reasons.

First, the major municipalities who were using trench safety plans likedthem and continued to use them. The inspectors had a useful tool in the formof a clear drawing to help them enforce the safe trenching intent of the law.Second, the revised OSHA Standard was enacted, which required thatemployers either used methods provided by the OSHA Standard, or by aRegistered Professional Engineer. Third, local prosecutors and plaintiff’sattorneys had made clear that they could likewise hold the offenders andthose who permitted them to work in violation of the law accountable.

Ken Oden, the Travis CountyAttorney, was also alarmed by thetrench fatality rates of the 1980’s. Helikewise noted that OSHA hadcriminally prosecuted only a handfulof employers in their history andimprisoned only one. It seemed clearto him that OSHA could not be reliedupon to prosecute the guilty so hetook it upon himself to do so. In1987, he and his assistant, AliaMoses, successfully prosecuted threecontractors whose unsafe trenchingpractices led to deaths on their jobs inthe Austin area: Sabine Consolidated,Peabody Southwest and Joe BlandConstruction. The appeals took somefour years to settle, but the State

House Bills NotWithout Problems

A County AttorneyTakes Action

Ken Oden, the TravisCounty Attorney, was

alarmed by thefatality trench rates

of the 1980’s. Itseemed clear to himthat OSHA could not

be relied on toprosecute the guiltyso he took it uponhimself to do so.

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finally decided that the prosecutions did not represent a pre-emption ofOSHA and were within the bounds of the County Attorney’s authority. Afinal appeal to the United States Supreme Court in December of 1992resulted in the convictions being allowed to stand. This sent a shock wavethrough the industry. Contractors who once thought they might be able tofold their tent and slip away to establish business under another name inanother location now knew they personally could face criminal prosecutionand imprisonment.

The plaintiff’s attorneys delivered the final blow. Trench fatalities had longbeen a favorite of plaintiff’s attorneys because employer negligence wasalmost always a given. Again, the investigation of the accidents showed that,in nearly all cases, no protection, or clearly inadequate protection wasprovided. Plaintiff’s attorneys found them simple to win. Those who hiredthe contractors typically felt that they were protected from prosecution bythe use of clauses in the contract indemnifying them from the actions of thecontractor. Much to their chagrin, they discovered that was not the case, asthe attorneys began to ensnare them in their nets, as well.

Carmen Mitchell, a Dallas based attorney who successfully sued acontractor and the City of Grand Prairie, Texas in the wake of a cave-in inwhich a sub-contractor’s employee was injured notes that,

What the record has clearly shown is that when conscientious owners hireconscientious contractors who use means and methods required by the law,the fatalities stop. If the disreputable find sanctuary where they are permittedto avoid their responsibilities, however, they continue to occur. Figure 5,below shows the record of trench fatalities in Texas prior to and after theefforts of local legislators, prosecutors, and the enactment of the revisedFederal OSHA Standard.

The Final Blow

“Regardless of how many times a witness recites theincantation that the contractor used his own ways meansand methods, he will not be believable if he contends thatthe city does not have some control over the contractor. Itsimply flies in the face of logic that a city paying hundredsof thousands of dollars for a job does not have the right tocontrol the party who is installing it”. (C. Mitchell “LegalAspects of Trench Failure)

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Trench Fatalities in Texas

House Bills House Bill Revised OSHA 662 and 665 1569 enacted Standard

enacted Sept. Sept. 1989 enforced 1987 Travis County Criminal Prosecutions undertaken

Figure 5

Sources:

1976-85State Dept. of Health

1986-87Travis County Attorney’s Office

1988-92TEEX – network

1993-2002Texas Worker’s Compensation Commission

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A Thumbnail Sketch ofthe OSHA Standard forExcavationsOSHA Standard 29 CFR 1926.650-652Subpart P

OSHA is the Occupational Safety and Health Administration, which is anagency within the Federal Department of Labor. They are charged withpromulgating and enforcing the safe work regulations, which are found inTitle 29 of the Code of Federal Regulations, which is the Labor title. Part1926 contains the regulations for the Construction Industry. Sections 650-652 contain the Excavation regulations, which are also known as Subpart P.

This standard applies to all open evacuations made in the earth’s surface.Excavations are defined to include trenches.

Any man made cut cavity, trench or depression in an earth surface, formedby earth removal.

A narrow excavation (in relation to its length). In general the depth isgreater than the width, but the width of a trench (measured at its bottom) isnot greater than 15 feet.

Each employee is an excavation five feet deep or deeper shall be protectedfrom cave-ins by an adequate protective system, unless the excavation ismade entirely in stable rock.

Under 5 feet, the requirement for a protective system is a judgment call forthe “Competent Person.”

Scope and Application

Excavation

Trench Excavation

General Requirement forProtection

Under 5 Feet

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Means “…one who is capable of identifying existing and predictablehazards in the surroundings or working conditions which are unsanitary,hazardous or dangerous to the employees and who has authorization to takeprompt corrective measures to eliminate them.”

Note: The primary candidate for the Competent Person role is the onsitesupervisor, backed up by the backhoe or excavator operator.

In the preamble to the Standard, OSHA says that, “… for the purposes ofthis standard, one must have had specific training in and be knowledgeableabout soils analysis, the use of protective systems and the requirements ofthe standard. One who does not have such training or knowledge cannotpossibly be capable of recognizing existing and predictable hazards inexcavation work or taking prompt corrective measures.”

In Subpart C, OSHA also requires that all employees be trained in the“recognition and avoidance of hazards”, so they will not unwittingly exposethemselves to unsafe conditions.

• Conducts tests for soil classification• Understand standards and any data provided• Determine proper protective system• Recognize and reclassify soil after changes in conditions• Determine whether damage to excavation safety equipment renders

it unusable• Conducts tests for hazardous atmospheres• Design of structural ramps*• Location of underground installations/utilities• Monitor water removal equipment and operation*• Perform daily inspections• Determine the necessity for a protective system if less than 5 feet

deep

It is the general responsibility of the Competent Person to ensure that allaspects of the excavation process are in compliance with the excavationstandard and the General Duty Clause of the OSHA Act (5a1.), whichrequires the employer to provide a safe, healthful workplace, free ofunknown or recognizable hazards.

Competent Person

Training Requirementsfor theCompetent Person

Specific Responsibilitiesof theCompetent Person

General Responsibilityof theCompetent Person

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1. Use the OSHA Standard for guidance with:• Sloping• Shoring with timber or aluminum hydraulic shoring• Shielding

2. Use a Registered Professional Engineer to provide:• Tabulated data• Manufacturers tabulated data• A site specific design ** Must be registered in the state where the work is being done.

• The excavation is deeper than 20 feet• An “alternate system” (such as sheet piling) that the Standard does

not provide guidance for is used• If the excavator is at “variance” with the Standard (i.e. doing less

than the standard requires)

Note: OSHA expects that the engineer will be registered in a related area,such a civil, mechanical, geotechnical, or architectural engineering.

Excavation protection solutions must either come from the OSHA Standardor a Registered Professional Engineer.

Optionsof theCompetent Person

Registered Professional Engineersmust be used if:

It is a‘Show Me In Writing’ Standard

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When OSHA revised the Excavation Standard, they knew full well thatmost jobs would not require a site-specific plan drafted by a registeredprofessional engineer. Many jobs would be fairly simple and straightforwardin nature. In those cases, OSHA allows the contractor to use tabulated ormanufacturers tabulated data (discussed above), or the resources of theExcavation Standard itself. Appendix A of the revised Excavation Standardprovides a soil classification system that allows the contractor to identify thesoil “Type” present on their site. They also provide Tabulated Data for theuse of sloping, timber shoring and aluminum hydraulic shoring, andguidance for the use of shield systems that would be used in conjunctionwith the engineered data provided by the manufacturer or designer of theshield.

OSHA enlisted the aid of the National Bureau of Standards to create theirsoil classification system. It was their intention to make the system as easyas possible for the non-engineer to use; literally, one as easy as ABC. Whatthey developed came to be known as the simplified soil classificationsystem, one that consists of 4 Types: stable rock, and Types A, B and C.Stable rock was the most stable type, followed in decreasing order by A, B,and C.

OSHA provided definitions for each soil type, as well as a series of manualand visual tests to be performed by the Competent Person to determine thetype (or types) of soil present on their site. The competent person is requiredto make at least one manual and one visual test to determine the soil type.Note: the competent person should not enter an unprotected excavation totest the soil. They should test freshly excavated material from the spoil pile.The competent person should be mindful that the soil type might changefrom the top to the bottom of the excavation, and from end to end. Theyshould test each different layer or zone to see if a change in the soilclassification is warranted.

Using the resourcesof theOSHA Standardin lieu ofengineered plans

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Once the competent person determines the soil “type”, they can turn to theother appendices in the Excavation Standard for specific guidance withsloping or shoring systems.

Stable rock is defined as natural, solid mineral matter that can be excavatedwith vertical sides and remain intact while it is exposed. As noted earlier,many believe rock to be stable simply because it is hard. That is simply nottrue. If the rock is fractured, faulted or fissured, if the rock consists of bedsthat are inclined steeply into the excavation, if some of the beds are greaselayers such as coal or mudstone, if water is seeping through the layers, itmight become unstable and fail as soon as the excavation is opened.Blasting and rock sawing might further destabilize the rock. It is for goodreason the departments of transportation place, “Danger, Watch Out forFalling Rock”, signs alongside roadways and use such methods as buildingfences or barrier walls at the base of rock faces to prevent their failure fromharming motorists.

Most rock has stability problems. The competent person should downgradeunstable, dry rock to Type B, and unstable wet or submerged rock to TypeC. If a contractor believes they truly have Stable Rock, as they might if theywere excavating a single, monolithic, horizontal bed or layer, they shouldstill examine it closely for evidence of weak zones, and consult ageotechnical engineer to confirm its stability, or obtain guidance for makingit so.

Defined as:

1. Cohesive soil with an unconfined compressive strength of 1.5 tonsper square foot or greater (these would be hard clays). Examples ofcohesive soils are clay, silty clay, sandy clay and, in some cases, siltyand sandy clay loams.

2. Cemented soils, such as caliche and hard pan, are also considered tobe Type A (because of their high unconfined compressive strength).

However, no soil is considered to be Type A if:

1. It is fissured

2. The soil is subject to vibration from heavy traffic, pile driving, orsimilar effects.

3. The soil has been previously disturbed.

Type A Soil

Stable Rock

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4. The soil is part of a layered system (cracks or fissures count aslayers) where the layers dip into the excavation on a slope of 4horizontal to 1 vertical or greater.

5. The material is subject to other factors that would require it to beclassified as a less stable material.

The exclusions for Type A typically eliminate the possibility of its existenceon a construction site. Practically speaking, B is the optimum soil type thancan be expected. Simply stated, B is the best it can be.

Defined as:

1. Cohesive soil with an unconfined compressive strength greater than.5 but less than 1.5 tons per square foot (this would be medium stiffclay).

2. Granular cohesion-less soils including angular gravel, silt, silt loam,sandy loam and, in some cases, silty and sandy clay loams.

Speed Shore Note: #2 is a troublesome definition. As will be seen inthe Type C definitions, most granular cohesion-less soils areclassified as type C and it is our view that all of them should be.Angular gravel may well stand at the slope profile dictated for TypeB soils by the OSHA Standard, but it cannot be shored in the mannerof a Type B soil by such systems as timber when gaps are left in theuprights or sheeting. Silty and sandy clay loams have enough clay toexhibit cohesive behavior and shouldn’t be classified as granularcohesion-less soils at all. Silt, the finest of the granular soils, willsometimes exhibits cohesive behavior and one particular kind of siltknown as a loess is characterized by its ability to maintain nearvertical slopes but is subject to failure when disturbed by excavatingor the imposition of surcharge loads. For these reasons, werecommend that all granular cohesion-less soils be classified as theworst case, Type C, in order to safeguard workers.

3. Previously disturbed soils except those that would be classified asType C soil.

Speed Shore Note: This is probably this most misunderstooddefinition in Appendix A. It is OSHA’s view that a previouslydisturbed soil never returns to its original state and should, for that

Type B Soil

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reason, be automatically downgraded to Type B soil. This is thereason they say Type B is a previously disturbed soil. The additionalverbiage that says “…except those that would be Type C soils”,refers to further downgrading the soil due to other reasons such asan unconfined compressive strength below .5 tsf or submersion. Acommon misperception is that all previously disturbed soil is Type Csoil. That is simply not what the definition says. A previouslydisturbed soil cannot be Type A. It is automatically downgraded toType B, but only further downgraded to Type C if other conditionsrequire it.

4. Soil that meets the unconfined compressive strength or cementationrequirements for Type A but is fissured or subject to vibration.

5. Dry rock that is not stable.

6. Material that is part of a sloped, layered system where the layers dipinto the excavation on a slope less steep that four horizontal to onevertical or greater, but only if the material would otherwise beclassified as Type B soil.

Defined as:

1. Cohesive soil with an unconfined compressive strength of .5 tsf orless (this would be a soft, wet, clay).

2. Granular soil including gravel, sand and loamy sand.

3. Submerged soil or soil from which water is freely seeping.

4. Submerged rock that is not stable.

5. Material in a sloped, layered system where the layers dip into theexcavation on a slope of 4 horizontal to one vertical or greater.

Practically speaking, there are only two soil types that can be present on ajobsite; Types B and C. Relatively stiff cohesive (clay rich) soils, orcemented soils that are free of fissures or layers dipping into the excavationat a slope of 4 horizontal to 1 vertical or greater are Type B soils. Loosegranular soils such as sand or gravel, soft wet clays, or cemented or clayssoils with fissures that dip into the excavation on a slope of 4 horizontal to 1vertical or greater are Type C soils.

Type C Soil

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In some cases, a competent person may have their soil type determined forthem. For example, it is common for a refinery located on the coast todeclare their site “C by Decree”, due to the presence of a water table nearthe surface, the effect of daily tides on the water content of the soil, and theenormous amount of soil mixing that occurs on such sites as a resultof continual maintenance and construction operations. Contractors workingin areas such as southern Louisiana where rain and groundwater arecontinually present often use protective systems for Type C soilautomatically; as they know they will encounter wet soil conditionsroutinely and plan for them from the outset. It is perfectly fine with OSHAfor the contractor to simply declare “worst case”, i.e. Type C, and goforward with protective systems designed for those soils.

If a contractor has a Type B soil, however, and may rely upon weather andwater conditions allowing it to remain Type B for the duration of the work,there is nothing wrong with declaring it Type B and using protective systemsintended for that Type. However, a competent person cannot just declare thesoil Type B; they must prove that it is Type B by performing a series ofmanual and visual tests on the soil to verify it.

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T he OSHA Standard provides guidance for the use of threeprotective systems: sloping, shoring (with timber and aluminumhydraulic shoring systems) and shielding. After determining the soiltype(s) on their construction site, the competent person can

refer to the Standard for guidance in the correct use of those systems.

Sloping prevents cave-ins by removing the unstable wedge of soil from thewalls of an excavation. Excavators are typically used to slope the walls backto a safe angle from which they will not fail. OSHA incorrectly used theterm “angle of repose” in the first excavation standard for that angle. Angleof repose is actually a stockpiling term. If dry soil is run up a conveyor anddropped, it will form a cone whose sides will automatically form the angleof repose. Since the term is properly a stockpiling term, OSHA substitutedthe term safe angle in the new standard. The safe angle is the angle at whichthe soil will stand. OSHA also uses the term maximum allowable slope,which is the maximum angle, measured from the horizontal, that they permitfor a given soil type, and actual slope, which is the actual angle, measuredfrom the horizontal, of the sloped wall.

The requirements for sloping are found in Appendix B of the excavationstandard, which is found elsewhere in this manual. The OSHA Standardprovides for four sloping options:

1. Slope the excavation 1 ½ horizontal to 1 vertical. In this case, thecompetent person will slope the excavation as though it were a TypeC, worst case soil. If the competent chooses this option, they are notrequired to classify the soil, as they will be defaulting to a worst casecondition.

2. Classify the soil using Appendix A of the excavation standard, andthen use the sloping chart and diagrams in Appendix B to design theslope.

3. Use tabulated data, created by a registered professional engineer, todesign the slope.

4. Use a site specific design prepared by a registered professionalengineer.

Sloping

Protective Systems:Sloping, Shoring & Shielding

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Option 1 is a common practice, particularly on the part of contractorswhose jobs are of sufficient duration to anticipate weather conditions turningthe soil to Type C during the course of the work. If the excavation is slopedfor Type C, they simply have to eliminate any accumulated water, inspectthe excavation to ensure the slope profile is performing as intended, andreturn to work. Likewise, some plant sites where continuous constructionactivity resulting in the mixing of a variety of soils over the years is coupledwith a high annual rainfall, ground water and tidal flows declare themselves“Type C by decree” to ensure that workers will always be safe by preparingfor worst case conditions.

Option 2 is the one most commonly used, especially when the contractor isconfident that weather and groundwater conditions are such that they mayrely upon the soil to remain the same “Type” for the duration of the job.Stable rock and Type A soil are not often found on construction sites, so thecontractor typically determines whether they have a Type B or C soil, thenrefer to the chart and diagrams of Appendix B to design their slope. It isimportant to remember that a single rain shower can turn Type B into TypeC soil by either causing it to soften into the unconfined compressive strengthrange of Type C soil, or become submerged, which is a definition of Type Csoil. In such an event, the contractor who had classified their soil as Type Band proceeded to slope on that basis would have to either wait until the soilreturned to a Type B condition by drying out, or re-slope their excavationfor Type C soil.

The OSHA standard assigns the following slopes to the four soil types:

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20’ Max.20’ Max.20’ Max.20’ Max.20’ Max.


The University of Texas at Arlington - Susan Harwood Program Grant 21.



OSHA provides guidance for the use of Timber Shoring in Appendix C.

Note #1: The timber must be full dimensional Oak or normal dimensionalDouglas Fir or equivalent.

Note #2: The shoring must be constructed using the dimensions and spacingrequired by OSHA’s Shoring Charts or one designed by a registeredprofessional engineering.

The OSHA Shoring Chart for Type B soil using Oak timbers is on the nextpage.

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Soil Type B Pa = 45 x H + 72 psf (2ft surcharge)

Figure 1: Timber Trench Shoring — Minimum Requirements2

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Depthof

Trench(ft)

Size (actual) and Spacing of Members1

Cross Braces Wales Uprights

Width of Trench (ft)HorizSpace

(ft)

VertSpace

(ft)

Size(in)

VertSpace

(ft)

Max Allowable Horiz Spacing (ft)

5to10

10to15

Up to4

Up to6

Up to9

Up to12

Up to15

Close 2 3

Up to6

Up to8

Up to10

SeeNote 1

Up to6

Up to8

Up to10

SeeNote 1

Up to6

Up to8

Up to10

SeeNote 1

SeeNote 1

4 x 6 4 x 6 6 x 6 6 x 6 6 x 6 5 6 x 8 5 2 x 6

6 x 6 6 x 6 6 x 6 6 x 8 6 x 8 5 8 x 10 5 2 x 6

6 x 6 6 x 6 6 x 6 6 x 8 6 x 8 5 10 x 10 5 2 x 6

15to20

Over20

6 x 6 6 x 6 6 x 6 6 x 8 6 x 8 5 8 x 8 5 2 x 6

6 x 8 6 x 8 6 x 8 8 x 8 8 x 8 5 10 x 10 5 2 x 6

8 x 8 8 x 8 8 x 8 8 x 8 8 x 10 5 10 x 12 5 2 x 6

6 x 8 6 x 8 6 x 8 8 x 8 8 x 8 5 8 x 10 5 3 x 6

8 x 8 8 x 8 8 x 8 8 x 8 8 x 10 5 10 x 12 5 3 x 6

8 x 10 8 x 10 8 x 10 8 x 10 10 x 10 5 12 x 12 5 3 x 6

(Footnotes)1 Mixed Oak or equivalent with a bending strength not less than 850 psi.2 Manufactured members of equivalent strength may be substituted for wood.



AluminumHydraulic Shoring

Aluminum hydraulic shoring is a system that uses extendable aluminumcylinders operated by fluid pressure to support the walls of an excavation.The walls of the excavation are literally jacked apart and supported by thecylinders. Invented in the late 1950’s, they were pioneered by the SpeedShore Corporation in the 1960’s. The light weight, ease of installation,multiple configurations and ability to compress the soil (which increased itsstability) resulted in hydraulic shoring becoming a very popular method.When OSHA revised the excavation standard, they decided to include twokinds of hydraulic shoring systems (vertical shores and horizontal walers) inwhat became Appendix D. The vertical shore mounts two cylinders inaluminum rails and is used in a vertical orientation. The horizontal walermounts two cylinders in aluminum rails used in a horizontal orientationwith sheeting behind the rails.

The manufacturers of aluminum hydraulic shoring all make their verticalshores and wales so that they can be used in the manner required by theStandard. Manufacturers also generate their own tabulated data to provideadditional options for vertical shores and walers, and for other systems notcovered by the Standard, such as single shores and 4-way manhole braces.The single shore is a single cylinder mounted to two short foot pads, or rails.The four-way manhole brace uses four, 2 or 3-inch cylinders mounted insteel sleeves which, when connected together, creates a 4-sided bracingsystem which can support square or rectangular excavations.

The Standard requires that the cylinders be a minimum of 2 inches indiameter, and have an axial compressional load rating of at least 17,000pounds. The aluminum rail to which the cylinders are attached must be6061t aircraft grade aluminum. Heavy-duty 3-inch cylinders can also beused to support trenches up to 15 feet in width.

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The OSHA Standard, and Speed Shore’s manufacturers tabulated dataappears elsewhere in the standard, but there are a number of points that bearconsideration here.

1. Hydraulic shoring is installed and removed from the top of theexcavation; no employee exposure is required. Special installationtools (hooks) are used to install vertical and single shores andrelease the hoses. Walers are typically installed by backhoes usingstandard rigging bridles.

2. The spacing of the cylinders is the critical factor. The standardrequires that the top cylinder on a vertical shore be no more than 18"from the top of the excavation. Manufacturer’s data allows the topcylinder to be between 12" and 24" from the top. There can be amaximum of 4 feet vertical spacing between the cylinders on avertical shore installation. The bottom cylinder on a vertical shorecan be no more than 4’ above the bottom of the trench. Thehorizontal spacing between the cylinders is a function of soil typeand depth and is found in the tabulated data.

3. For waler systems, the top cylinders, or center line of the rail, can beno more than 2 feet below the top of the trench. There can be amaximum of 4’ vertical spacing between the waler systemsvertically, and the bottom waler can be no more than 4 feet abovethe bottom of the trench. The “section modulus” column of the walercharts in the OSHA Standard refers to the size of the rail in crosssection and corresponds to light, standard and heavy-duty rail. Asthe rail size increases, the horizontal spacing between the cylindersalso increases.

4. OSHA requires 3"x12" timber uprights as sheeting for walersystems, but manufacturers provide alternatives such as aluminumand fiber composite materials.

5. There is a 20,000 pound surcharge load limit from equipmentalongside the excavation when OSHA’s charts are used.

6. Three vertical shores, evenly spaced in a group and equallypressurized are required by OSHA to make a system, butmanufacturers’ data will allow the use of two.

7. The fluid used in these systems is a bio-degradable, environmentallyfriendly formulation.

8. The Standard provides charts for the use of vertical shores in TypesA and B soils, and the use of waler systems in Types B and C.Manufacturers will allow the use of vertical shores in the betterrange of Type C soils, known as C-60 soil. For a full explanation ofC-60 soil, please refer to the manufacturer’s tabulated data sectionof this manual.

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9. Plywood may be used for sheeting to control local raveling, orsloughing, of the trench face, but it is not intended as a structuralmember. In other words, it is not a requirement to use it if there is noraveling of the trench wall between the shores. If plywood is used,however, it must be 1 1/8" thick softwood or ¾” hardwood (Fin-Form) plywood. Manufacturers’ data provides alternative sheeting,including two sheets of ¾” softwood plywood.

The question is often asked, “Why is no sheeting required in betweenadjacent vertical shores? Won’t the soil fall out between them?” Theanswer is no. When the cylinders are pumped out and the soil iscompressed, the soil becomes more stable due to a principle known assoil arching. If you have ever observed brick masons carrying brick in abrick clamp, you know that as long as the lateral pressure exerted by theclamp exceeds the pull of gravity, the row of brick will not fall out of theclamp.

The lateral pressure can be applied at a more oblique angle to producethe same result. Consider the silo. Sometimes, when the gate is opened,the material is too tightly compressed by the configuration of the silo toallow it to flow out.

When the shores are pumped out against the trench walls, they producethe same effect. They not only push directly into the walls, but atangles. The space between the shores where the soil is being compressedat an angle corresponds to the space at the gate of the silo.

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One might ask, “That’s fine for the face of the wall, but how about theweight of the wall bearing down behind the face, particularly as you getdeeper?

That’s the other interesting aspect of soil arching. When the cylinderscompress the soil, they create 360 degree compression cones in the wallsthat link up to create what appears to be a structure much like an archedbridge.

When a load is imposed on an arch, the arch transfers the load to thelegs. The compression cones likewise transfers the load of the wall tothe cylinders. It is on the cylinders, not in the gaps between them, thatthe load rests.

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Shield Systems

Shield systems differ significantly from sloping and shoring systems in thatthey are not designed to prevent cave-ins, but rather to protect the workerswhen a cave-in occurs. In a real sense, they function like bomb shelters.They are structures, typically “box-like” in design, that are placed in theexcavation to provide worker protection. They typically consist of two flat,parallel walls supported by cross braces (known as spreader bars). If a pointrepair is required, the contractor simply excavates around the damaged area,and lowers the shield in on top of the pipe to provide a safe workplace. Ifinstallation of a new pipe service is required, the contractor excavates thearea for the installation of the first joint, and then lowers the shield in place.After the first joint is installed and aligned, the excavator backs up,excavates in front of the shield, then pulls it forward by either pulling on thespreader bars, or a pulling bridle attached to pulling lugs on the front of theshield. After the next joint of pipe is installed, the process is repeated.Contractors typically cover the pipe with the fill material as the box ispulled forward. This is what is known as a cut and cover operation. At theend of the day, the trench is closed up to the shield, keeping the right of wayrequirements minimized. In unstable soils, the contractor may have to diguntil the soil begins to fail, then install the shield and force it downward bypushing on the pounding blocks atop the side-walls as they excavate todeeper levels.

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Shield Systems



Shields have become the preferred means of providing worker protectionwhen possible, due to their ease of use, the minimum time required to installmove and remove them, the minimum right of way requirements, and thefact that they can be used in unstable soils where vertical walls can’t berelied upon.OSHA does not provide an Appendix for the use of shield systems. Whatguidance they do provide is found in the General Requirements forProtective Systems in 1926.652. Those paragraphs require the following:

1. Shields must be designed by a registered professional engineer. Acopy of the design, with the identity of the engineer who provided itmust be made available to OSHA upon request. Few contractorsdesign and build their own shields. Most rent or buy them frommanufacturers who provide written certifications for their products.The certifications may also appear in the form of plates affixed tothe shield. OSHA compliance officers like to match the numbers onthe shield to the numbers on the certification documents.

2. Shields cannot be used in excess of the loads they are rated for.Never use the shield at a depth greater than the rating provided bythe certification. Never increase the height or length of the shieldwalls by installing metal plates above, below, or behind them unlessa registered professional engineer has approved it in writing. Bewary of shields that limit the use of the shields to anything other thanC or C-80 soils or do not permit its use in a trench for over 24 hours.In most cases, contractors expect their shields to stay in use until thejob is complete, and to bear worst case weather conditions (read Csoils).

3. You cannot make any additions to, any subtractions from, or anyalterations to any shield without written consent from themanufacturer or designing engineer. This includes stacking pins forstacker shields and keeper pins.

4. Any structural damage, such as bending a spreader bar or breaking asocket weld, is cause for taking the shield out of service until repairshave been made in accordance with manufacturer’s guidelines andrequirements.

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5. Shields must be installed in a manner to restrict lateral movement.While OSHA does not officially permit a gap between the shield

wall and the excavation, they are aware that the contractor intendsto raise, lower and pull the shield, so they “unofficially” allow about4" as long as there is no danger of an employee being injured if thebox were to shift the distance of that gap if a cave-in were to occur.

6. If the shield does not reach the top of the excavation, the sectionabove the shield must be sloped to a point at least 18" below the topof the shield, or, if the shield is designed for use in a stackingconfiguration, a second shield may be stacked atop it.

7. Workers may remain inside a shield when it is pulled forward, butthey cannot remain in it when it is installed, removed, or raisedvertically.

8. Workers must enter and exit the shield in a protected area, which isto say a ladder used for that purpose must be inside the shield, notoutside it in the unprotected trench.

9. Shields may be raised two feet off the bottom of the trench as long asit is rated for the full depth of the excavation, and as long as no soilcaves out from under it.

10. The workers must not be exposed to walls at the end of the trench.If the trench cannot be sloped in front of the shield, end plates mustbe installed to protect the workers from cave-ins at the open end.

Shield systems come in an enormous variety of sizes and configurations.Lightweight shields may be constructed of aluminum or fiber compositematerials. Some shields, known as manguard rings, are round in shape.Shields may have cut out panels to allow pipe to be installed through thesides or under closed end plates. Other may have special high clearancespreader bars or arched spreaders to allow for clearance over large diameterpipe. Manufacturers continue to modify and create shield systems to meetthe needs of constructors.

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Shield Installation


The University of Texas at Arlington - Susan Harwood Program Grant32.

excavation safety manual

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